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I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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LUMINOSITY MEASUREMENT AT ILC
I. Bozovic-Jelisavcic(on behalf of the FCAL Collaboration)
Vinca Institute of Nuclear SciencesBelgrade, Serbia
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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FCAL Collaboration National Center of Particle & HEP, Minsk, Belarus
LAL Orsay, France
Royal Holloway University of London, Great Britain
DESY, Hamburg & Zeuthen, Germany
Tel Aviv University, Israel
KEK, Japan
Tohoku University, Japan
AGH University, Krakow, Poland
Jagiellonian University, Krakow, Poland
Institute of Nuclear Physics, Krakow, Poland
University of Warsaw, Warsaw, Poland
Joint Institute Nuclear Research, Dubna, Russia
IFIN-HH Bucharest, Romania
VINCA Inst. of Nuclear Science, Belgrade, Serbia
CERN, Switzerland
Argonne National Lab, Upton, USA
University of Colorado, Boulder, USA
University of Santa Cruz, USA
SLAC, USA
Institutes contributing to LumiCal related studies
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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Luminometer at ILC
Distance from IP [m] 2.5
Geometrical aperture [mrad] [31,78]
Fiducial volume [mrad] [38,69]
Number of layers 30 W/Si
Moliere radus [cm] 1.5
Sensor azimuthal/radial divisions 48/64
Resolution in polar angle [mrad] (2.2±0.02) 10-2
Energy resolution [GeV1/2] 0.21
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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LumiCal simulation study
LumiCal fiducial volume2.1 nb integrated x-sec.
3/1~ B
410
annual
statL
L
Esh [GeV]
Stable sampling term vs. shower energy
Esh [GeV]
Intergrated deposited E vs. E shower
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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Luminosity measurement
IT IS COUNTING EXPERIMENT, BUT...
B
thNL
int
B
i
coriNN
L
exp
int
real experiment 1. To build a device capable of precise reconstruction of E,
2. To control (other) systematics
Integrated luminosity can be determined from the total number of Bhabha events produced in the acceptance region/fiducial volume of the luminosity calorimeter and the corresponding theoretical cross-section
Event selection
1. the polar angle of the reconstructed shower must be within the detector fiducial volume at one side and within at the other.
2. total energy deposited in the LumiCal must be more than 80% of the center-of-mass energy
mradmrad ff 7,4 maxmin
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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Luminosity measurementSYSTEMATIC EFFECTS
Shower develops under a non-zero angle with respect to the probing geometry bias in luminosity measurement
TO BUILD A DEVICE…
4106.1,8.0
L
Lmradl
reconstruction
BhabhaEofcontrolL
L )3(4)3(4 10210
=3.2∙10-6 rad
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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More systematics
TO BUILD A DEVICE…
Energy resolution
%5.110 4
L
L control of the sampling
term E (also called _res at slide 4) , or 25% for
OTHER SOURCES OF SYSTEMATICS
2- background
High x-sec 10s nb, spectators close to the beam pipe
310L
L
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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More systematics
However, less than 1% of spectators in the LumiCal
B/S ratio saturates within the same order of magnitude at ILC energies B/S=2.3 10-3 at 500 GeV and B/S = 5.2 10-3 at 1 TeV
Sensitivity of background to signal ratio to systematic effects that may come from the uncertainty of detector fiducial volume due to various detector displacements is negligible
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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More systematics
Space charge effects
Beam-beam interactions
Modification of initial state: Beamstrahlung √s’≤√s, ini≠ 0, Eelec≠ Eposit
Modification of final state: Electromagnetic deflection Bhabha angle reduction (~10-2mrad) + small energy losses
Total BHabha Suppression Effect (BHSE) ~1.5%
Background hits on the front plane before and after selection applied
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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More systematics
Simulation of BHSE measurement
BHSE from BS can be exp. measured and treated as a bias
However, to provide (BHSE) ~0.4%(0.1%) beam parameters x
and z have to be known within
20%(5%).
Data-driven method from reconstructed luminosity spectrum by measuring anglesin the LumiCal
Dominant effect comes from beamstrahlung
BS+EMBSNO BHSE
Impact of beam-beam effects on precision luminosity measurements at the ILC, C. Rimbault et al., JINST 2:P09001,2007
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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More systematics
x=755 nmx=555 nm
Bunch width variation for ±100 nm around nominal value x=655 nm
Test beam studies are needed to determine experimental uncertainties of effects that should be taken as corrections (i.e. bias in polar angle).
Precision determination of Bhabha energy and understanding of detector energy resolution is necessary due to the applied selection.
NLO calculations at ILC energies are needed both for Bhabha and background processes.
Dominant effects come from beam-beam interaction (BHSE) and 2- processes. Both can be corrected for. In BHSE case the correction is large and require beam parameter control at 20% level or better (BS component), while uncertainty in physics background comes from the error on x-section.
SUMMARY ON SYSTEMATICS
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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Total systematics
TOTAL SYSTEMATICSat 500 GeV
Value Relative uncertainty of the value
L/L
x-secBhabha [nb] 2.1 5.4 10-4 ** 5.4 10-4
[mrad] 2.2 10-2 100% * 1.6 10-4
[mrad] 3.2 10-3 100% * 1.6 10-4
res [GeV-1/2] 0.21 1.5% 1.0 10-4
EBhabha [GeV] 200 2.0 10-4 1.0 10-4
B/S 2.3 10-3 100%* 2.3 10-3
BHSE [%] 1.51 9.9% *** 1.5 10-3
2.8 10-3
* Upper limit – the size of effect is taken as uncertainty.** Uncertainty of the theoretical cross-section for Bhabha at LEP energies [OPAL, G. Abiendi et al., Eur. Phys. J C14(2000)373].*** 5% control of bunch x and z sizes.
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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Summary
It has been proven through simulation that it is possible to design luminometer at ILC capable of precision reconstruction of Bhabha energy and polar angle.
Numerous systematic effects are present and understood (again) at the level of simulation. They amount to 2.8∙10-3 systematic uncertainty in luminosity at the upper limit, with the statistical error on luminosity less than 10-3 needed for integrated annual luminosity at high energies.
Most of systematic effects can be taken as corrections once their experimental (test-beam) or theoretical uncertainties are known.
The largest contribution to the relative error on luminosity comes from collective (beam-beam) effects and physics background. According to the present knowledge, beam-beam effects can not be reduced below 10-3 (even if bunch size is controlled at 5% level). However, it doesn’t relax the need for detector precision.
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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To do list
Sensitivity of the luminosity measurement to changes of the detector fiducial volume implies importance of mechanics and position control of the LumiCal (inner radius, various radial displacements, F-B relative positions, etc.). It is needed to quantify impact of these effects on luminosity measurements within the current detector geometry, as well as to prove in situ mechanical control.
Test-beam studies are needed to understand experimental uncertainties of some effects (i.e. realistic calibration procedure).
Space charge effects introduce error in luminosity measurement of order of 10-3. They have to be studied in more details with respect to changes in geometry and at all ILC energies*.
Finally, for the reason of completeness, theoretical uncertainties at the NLO level are needed for Bhabha and background processes at ILC energies*.
Final choice of shape and material of the beam-pipe has to be simulated to estimate impact of pre-showering on luminosity measurement. For parallel vs. conical beryllium pipe the effect is estimated to be O(10-4).
* the same is true for CLIC
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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BACKUP
I. Božović-Jelisavčić, LCWS10, Beijing, 26-30 March 2010
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Mechanical issues
Systematic impact on luminosity measurement
IN SITU
• LPS prototype monitors LumiCal as a whole object
• Obtained accuracy 0.5m in the X-Y plane and 1.5m in z direction – order of magnitude better than required
• Method for measuring displacement of individual sensor layers/inner radius under study
All by A.Stahl, old geometry [26,82] mrad, 3,05 m from IP
100 m for L/L~10-4, ~ 1.5 mm for L/L~10-3
Error in half-
4 m for L/L~10-4, ~40 m for L/L~10-3